Planar fuel cell and method for the production thereof

ABSTRACT

A planar fuel cell comprising an electrode-membrane-electrode assembly, wherein the membrane includes a fabric having a warp fibers which are continuous insulating fibers of an electrically insulating material and weft fibers comprising of both fibers of the insulating material and fibers of an electrically conducting material in an alternating fashion, so as to form insulating areas and conducting areas, respectively.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority based on International PatentApplication No. PCT/FR2004/050109, entitled “Planar Fuel Cell and Methodfor the Production Thereof” by Renaut Mosdale, which claims priority ofFrench Application No. 03 50051, filed on Mar. 18, 2003, and which wasnot published in English.

TECHNICAL FIELD

The invention relates to a planar fuel cell and to the method for makingsuch a cell.

The field of the invention is that of planar fuel cells, for examplewith a solid polymer electrolyte, and of their application to generatingelectrical powers from a few hundreds of milliwatts to a few hundreds ofkilowatts, for stationary applications, for example, for power stationsor boilers, applications to transportations, for example for land, sea,or air-borne vehicles, and portable or transportable applications forexample, for portable telephones or computers.

STATE OF THE PRIOR ART

At the present time, most fuel cells are built on the basis of asandwich assembly consisting of two electrodes positioned on either sideof an electrolyte. These electrodes generally consist of a diffusionlayer on which an active layer is deposited (catalyst layer). Adifferent reagent arrives on each external surface of both electrodes,i.e., a fuel and an oxidizer. The latter chemically react via theelectrolytic component so that it is possible to pick up an electricalvoltage at the terminals of both electrodes.

If the fuel is hydrogen, and the oxidizer oxygen, oxidation of hydrogentakes place at the anode, whereas reduction of oxygen into water occursat the cathode.

Each electrode is therefore the centre of an electrochemical reaction,the resulting voltage, the potential difference between both of thesereactions, is generally about 1 volt (with zero current) as oxidation ofhydrogen into protons is achieved at the anode and reduction of oxygeninto water is achieved at the cathode. This low voltage is the mainhandicap of such cells relatively to standard batteries, for which theelementary voltage may rise up to 4 volts (for example the Li/C pair).To find a remedy to this problem, a large number of such components areusually stacked according to a so-called filter press technology. Butthis technology has a problem of poor distribution of the gases in eachcell and of leaking in the stack, worsened by the multiplication of thenumber of stacked components. Moreover, the bipolar plates separatingtwo elementary cells must meet the following specific physical andchemical criteria:

-   -   very good electronic conductivity,    -   impermeability to gases,    -   low mass,    -   chemical resistance to water, oxygen and hydrogen,    -   low cost material,    -   good machinability.

No bipolar plate technology meets such criteria today, which requiresthe use of an expensive machining technique, or the use of very costlymaterials. Moreover, this type of stacking is generally of aparallelepipedeous geometry unpropitious to integration.

In order to overcome such drawbacks, a geometry for a fuel cell withwhich several pairs of electrodes may be associated on a same membraneand the elementary voltage may be increased artificially. Thisassociation is achieved by stacking materials shifted relatively to eachother. It requires the use of electronically insulating gas distributionplates.

As illustrated in FIG. 1, such a fuel cell consists of an assembly ofseveral individual cells 10, positioned either near or behind eachother, each comprising an anode 11 and a cathode 12, tightly enclosingan electrolytic layer 13. These individual cells 10 are separated fromeach other by insulating areas 17, and are connected with each other byconducting parts 14, a first end 15 of a conducting part 14 beingconnected to the cathode 12 of a first stack 10, and a second end 16 ofthis conduction part 14 being connected to the anode of another cell 10which is adjacent to it.

Such an assembly is difficult to produce, not only for making differentindividual cells 10 at a small scale, but also for making theirelectrical connection. Moreover, leak-tightness problems continue toexist.

To find a remedy to these drawbacks, the a prior art method is disclosedfor making an assembly of basic fuel cell components by forming severalelementary cells, by depositing on an insulating weft, in severalsuccessive steps, different components as suspensions.

FIG. 2 illustrates such an assembly of basic components, once finished.All the functional components of this assembly are parts deposited oneafter the other on and/or in a weft material plate, the thickness ofwhich corresponds to the thickness of an ion-conducting layer. First ofall, this assembly comprises a peripheral gasket 21, placed over thewhole thickness of the plate at the periphery of the latter. Thisperipheral gasket 21 is in a chemically inert and electronically andionically insulating material. These different elementary cells of thisassembly each consist of an anode 22 placed on a first surface of theplate, a cathode 23 placed on the opposite surface of the plate and aion conductor 24 located between the anode 22 and the cathode 23, overthe whole thickness of the plate. The anode 22 protrudes on one side ofthe ion conductor 24 and the cathode 23 protrudes from the ion conductor24 on the opposite side to the anode. In this way, each protrudingportion of an anode 22 and of a cathode 23 is found facing, within thethickness of the plate, a cathode 23 or an anode 22 of a neighboringcell, except for the anode 22 of a first end cell and the cathode 23 ofthe other end cell. An electron conductor 26, deposited over the wholethickness of the plate, enables the anode 22 of a cell of rank n to beconnected to the cathode 23 of the neighboring cell of rank n+1, whichis placed facing the latter, the voltage Ui (0<i<5) of the one beingtransferred to the other. Vertical insulating layers 25 separate eachelectron conductor 26 from both portions of the ion conductor 24 whichare adjacent to it. The distance a between both neighboring verticalinsulating layers 25 may be of the order of 5 millimeters. A firstelectron collector 27 is placed on the anode 22 protruding from a firstend cell and a second collector 27 is placed on the cathode 23protruding from the other end cell.

The major problems encountered in elaborating this type of planar fuelcell are the leak-tightness of the ion conductor/electron conductormaterial interfaces on the one hand, and the low values of electronconductivity obtained in the <<current crossings>> on the other hand.These low conductivity values cause high ohmic drops inducing losses ofperformances and heating of these crossings (Joule effect).

The object of the invention is to solve such problems.

DISCUSSION OF THE INVENTION

The invention relates to a planar fuel cell including anelectrode-membrane-electrode assembly, characterized in that themembrane includes a fabric, the warp fibers of which are continuousfibers in an electrically insulating material and the weft fibersalternately are fibers in insulating material and fibers in anelectrically conducting material, so as to form insulating areas andconducting areas, respectively.

Advantageously, the fibers in insulating material may be a polymer or aninert glass. The fibers in the electrically conducting material may becarbon fibers or stainless steel fibers.

Such a cell notably has the following advantages:

a simplification in the making by suppressing the step for depositingvertical insulating layers,

an increase in performance by providing a massive electron conductor inthe electrical crossings,

a size of electron conductors allowing the number of pairs of electrodesto be increased on a same surface, thereby increasing the voltage of thecell.

The invention also relates to a method for making a planar fuel cell,which comprises the following steps:

cutting out, with the desired shape, of a piece of material,

depositing a seal layer over the whole thickness of the peripheral layerof this piece of material with a slight excess thickness,

depositing a ion conductor over the whole thickness of this piece ofmaterial,

depositing anodes on a first surface of the thereby filled piece ofmaterial and cathodes on the other surface of the latter,

depositing electron collectors at one of the two ends of the assembly ofanodes and at the other end of the assembly of cathodes,

characterized in that the piece of material is a piece of fabric, thewarp fibers of which are continuous fibers in an electrically insulatingmaterial and the weft fibers alternately are fibers in insulatingmaterial and fibers in electrically conducting material to forminsulating areas and conducting areas, respectively.

Advantageously, an insulating gasket is deposited on either side of eachconducting area.

Because of the woven structure of this piece of fabric, the fibers arein intimate electrical contact, unlike the devices of the known artwhere conducting grains are embedded in a binder and where electricalcontinuity is not absolute. This piece of fabric therefore causes anincrease in conductivity by a factor 2 to 10 so that the performances ofthe cell may be improved and the size of the insulating areas andtherefore those of the cells may be reduced.

The applications aimed by such a type of (monoblock or polycomponent)fuel cell technology are light and portable systems, requiringelectrical voltage supplies larger than 1 volt, and in which problems ofweight and shapes are posed.

The fuel feeding a thereby built cell may be stored as compressed gas onthe outside of the cell or else in an adsorbed form in hydrides, whichmay be made as hydride sheets in contact with the anodes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a first device of the known art.

FIG. 2 illustrates a second device of the known art.

FIG. 3 illustrates a locally conducting weft according to the invention.

FIGS. 4 to 6 respectively illustrate the steps of the method of theinvention both in a transverse sectional view (FIGS. 4A, 5A and 6A) andin a top view (FIGS. 4B, 5B and 6B).

DETAILED DISCUSSION OF PARTICULAR EMBODIMENTS

In making juxtaposed planar cells of the known art, as illustrated inFIG. 2, a porous matrix is used which is locally filled with a suitablematerial for the functions which the filled area should perform. Such asolution has leak-tightness problems at the interfaces of the differentareas. Moreover, the electronic conductivity is not necessarily highbecause of the actual structure of the porous material.

To overcome such drawbacks, the invention consists of using, instead ofthe porous matrix, a fabric 30 of fibers in one piece. As illustrated inFIG. 3, the warp fibers 31 are continuous from one end to the other ofthe cell (there is no interface, therefore no loss of space) and are inan electrically insulating material. The weft fibers are made withinsulating fibers 31′ or conducting fibers 32 alternately so as toachieve the different functions of a cell component and to juxtapose thecomponents in order to form a cell.

The insulating fibers 31 and 31′ for example are polymer or chemicallyinert glass fibers. The conducting fibers 32 for example are carbon orstainless steel fibers.

These conducting fibers 32 form, as illustrated in FIG. 3, conductingareas 33 in order to locally provide over a width b, an electronicconductivity in the thickness of the weft. This width b may be of theorder of 2 millimeters, for a fabric surface area of about 1 squaremeter, and a thickness between 20 micrometers and 100 micrometers.

As illustrated in FIG. 3, this fabric 30 may be made with a standard webwoven at right angles. The number of fibers and the weaving angle mayvary depending on the selected geometry for the cell.

The method for making such a planar fuel cell comprises the followingsteps:

cutting out, with the desired shape, the piece of fabric 30 whichincludes insulating areas 34 separated by conducting areas 33,

depositing a seal layer 40 over the whole thickness of the periphery ofthis piece of fabric 30 in a slight over thickness,

depositing a ion conductor 41 over the whole thickness of this piece offabric 30

depositing the anodes 44 on a first surface of the thereby filled pieceof fabric and the cathodes 45 on the other surface,

depositing (not illustrated in FIGS. 4-6) electron collectors at one ofthe two ends of the assembly of anodes 44 and at the other end of theassembly of cathodes 45.

The different deposition steps provided above are advantageouslyachieved by means of masks.

In order to improve operation, by avoiding any ionic leak, it ispossible to deposit insulating gaskets on either side of each conductingarea 33.

Thus, according to these steps illustrated in FIGS. 4-6, it is possibleto make planar fuel cells having performances superior to those obtainedwith the cells described in the prior art, and reinforced mechanicalstrength by suppressing a ion conductor/electron conductor interface,also limiting the risks of internal leaks which may causehydrogen/oxygen mixtures.

The cell structure of the invention as shown above is only an example.The invention may be applied to microcells for example die-stamped on asupport or to all cells which have on a same plane, separateelectrically conducting surfaces and ion conducting surfaces.

1. A planar fuel cell comprising: an electrode-membrane-electrodeassembly, wherein the membrane includes a fabric having a warp fiberswhich are continuous insulating fibers of an electrically insulatingmaterial and weft fibers comprising of both fibers of the insulatingmaterial and fibers of an electrically conducting material in analternating fashion, so as to form insulating areas and conductingareas, respectively.
 2. The fuel cell according to claim 1, wherein thefibers in the insulating material are a polymer or an inert glass. 3.The fuel cell according to claim 1, wherein in the fibers inelectrically conducting material are carbon fibers or stainless steelfibers.
 4. The fuel cell according to claim 1, further comprisinginsulating gaskets disposed between the conducting areas and theinsulating areas.
 5. The fuel cell according to claim 1, furthercomprising a seal layer disposed around a periphery of the fabric,wherein the seal layer has a thickness larger than a thickness of thefabric.
 6. The fuel cell according to claim 1, further comprising ananode disposed on a first side of the fabric and a cathode disposed on asecond side of the fabric opposite to the first side.
 7. The fuel cellaccording to claim 1, further comprising an ion conductor disposed overthe fabric.
 8. A planar fuel cell including a fabric, the fabriccomprising: a plurality of insulating warp fibers oriented in a firstdirection, the insulating warp fibers being made of an electricallyinsulating material; and a plurality of weft fibers in the fabric andoriented in a second direction substantially perpendicular to the warpfibers, the weft fibers being made of an electrically conductingmaterial in a first conducting area, the weft fibers being made of anelectrically insulating material in a first insulating area, wherein thefirst conducting area and the first insulating area are adjacent to oneanother in the fabric.
 9. The fuel cell according to claim 8, furthercomprising the weft fibers being made of the electrically insulatingarea in a second insulating area located adjacent to the firstconducting area and on an opposite side of the first conducting areafrom the first insulating area.
 10. The fuel cell according to claim 8,further comprising insulating gaskets disposed between the firstconducting area and the first insulating area.
 11. The fuel cellaccording to claim 8, further comprising a seal layer disposed around aperiphery of the fabric, wherein the seal layer has a thickness largerthan a thickness of the fabric.
 12. The fuel cell according to claim 8,further comprising an anode disposed on a first side of the fabric and acathode disposed on a second side of the fabric opposite to the firstside.
 13. The fuel cell according to claim 8, further comprising an ionconductor disposed over the fabric.